Coating baths for aluminum and method of and materials for preparing and replenishing same



CQATHNG BATH FGR ALUMlNUh I AND METHOD F AND MATERIALS FQR PREPARING ANDREPLENESHHNG SAME Nelson 31'. Newhard, lira, Oreland, and David Y.Dollman, Lansdaie, 0a., assignors to Amchem Products, inc, Ambler, Pa, acorporation of Delaware No Drawing. Application January 2, 1959 SerialNo. 784,420

6 Claims. (Cl. 148-616) This invention relates to the art of applying achemically bonded coating to the surfaces of aluminum or alloys thereofin which aluminum is the principal ingreclient.

Before enumerating the objects and advantages of the invention, it willbe helpful to refer briefly to certain coating procedures which are nowfamiliar to those skilled in this art.

It is well known that aqueous acid solutions Whose principal andessential coating producing ingredients are fluorides, hexavalentchromium and phosphates are capable of producing decorative as Well ascorrosion resistant coatings on aluminum and typical examples ofprocesses which employ such solutions are described in US. Patents2,438,877, 2,472,864, 2,678,291 and 2,814,577.

Examination of the first three patents just mentioned will show that thesolutions employed in the procedures therein disclosed have chemicalconstituents which are present in amounts corresponding to thefollowing: Fluoride-0.15 to 12.5 grams/liter; hexavalent chromiumexpressed as CrO 3 to 60 grams/liter; and phosphate (PDQ-2 to 285grams/liter. These patents also show that the fluoride content of thebath should be maintained within certain limiting ratios to thehexavalent chromium content.

The fourth. patent listed discloses that, under certain conditions, itis possible and desirable to control the fluoride content of the bathwithout recourse to maintaining a definite fluoridezchromate ratio. Italso describes a method for the treatment of large areas of aluminumsurfaces in a manner which will yield substantial uniformity of colorand surface appearance which is an advantage of considerable moment inthe architectural field where substantial uniformity of appearance isimportant.

In commercial application coating baths such as just described operatequite satisfactorily within a pH range of from 0.8 to 4.5.

Beyond the foregoing the solutions used in the methods just referred toare otherwise very important commercially and for quite a number ofyears past there has been, in many industries, a growing demand foraluminum the surface of which has been coated in the mannerdescribedbecause of the decorative and protective values which can be imparted tothe surface by such coatings.

However, certain difliculties have been encountered some of which arisewhen the coating solutions are first prepared and others of which appearduring extended or continuous use of the solutions. For example, in theprocesses of the prior art referred to the solutions when first preparedare sometimes erratic in behavior in that they produce coatings ofvarying color which sometimes tend to have an overburden of powder. Theproblem of powder also tends to occur during continuous use of thecoating solutions especially when they are employed in the treatment ofrelatively large areas of alu- 2,936,254 Patented May 10, 1960 minumsurfaces. Ordinarily the powdery deposit formed on top of the coatingdoes not impair the corrosion resistance of the surface although itdefinitely impairs the decorative and paint bonding value.

In the prior practice referred to still another problem develops whichshould be understood in connection with the objects of the presentinvention. In the coating of aluminum surfaces some aluminum isdissolved, of which a portion becomes a part of the deposited coating,the rest dissolving in the coating solution. Since the concentration ofdissolved aluminum has been found to affect the coating properties ofthe solution, we have devised means to keep its concentration reasonablysmall and sensibly constant, because, by so doing, we find that uniformcoatings can be obtained on a succession of work pieces.

With all of the foregoing in mind the principal objects of the presentinvention are the provision of the following:

(1) A method of preparing an aluminum coating solution of the typedescribed which, from the time the very first piece of aluminum istreated by the solution, is capable of producing coatings which areuniform in color and without an overburden of powder.

(2) A method of and materials and solutions for coat ing a longsuccession of aluminum pieces by means of which it is possible tocontinuously produce satisfactory coatings both as to color uniformityand corrosion resistance.

How the foregoing, together with such other objectives as may appearhereinafter, are achieved will now be described.

Our invention is based upon the discovery that baths of the characterdescribed can be rendered capable of producing uniformly colored,powder-free coatings from the beginning if they are made up andreplenished in a definite manner with respect to the concentration offluoride and alkali metal therein. According to our in vention a bath ismade up and maintained by attention to the following details:

A. The baths must contain definite amounts of a mixture of sodium andpotassium in relation to the fluorine present.

B. The baths must contain dissolved aluminum in an amount limited by thesolubility of a solid phase of that alkali fluo aluminate, inequilibrium with the coating bath, Which is characterized by a contentof 6 fluorine atoms for each atom of aluminum. 1

C. Alkali metal in the bath must be in the proportion of 0.0026 to0.0526 mol of sodium plus potassium for each gram of fluoride, and ithas been found that of the total molar content of sodium and potassium,from 10 to should be potassium. Best results have been obtained wherethe total number of mols of sodium and potassium is from 0.0065 to0.0455 for each gram of fluoride present. In replenishing a coating bath(and particularly those subject to heavy loading) the alkali metalcontent should be restored in the approximate ratio of two mols ofpotassium for each mol of sodium and, for each gram of fluoride added,the total alkali metal added should be from 0.0065 to 0.0455.

As previously described, dissolved aluminum in the baths of the priorart adversely afiects the coating properties. By suitable make up andreplenishment in accordance with the specific procedures outlined above,the amount of dissolved aluminum in the bath is limited by the sparingsolubility of the alkali fluo-aluminate, due to the proper concentrationof sodium, potassium and fluoride as described above. The sparinglysoluble fiuo-' aluminate is found to have a composition in which, forone atom of aluminum, there are 6 atoms of fluorine. When either sodiumor potassium was the only alkali metal present in the precipitate the.bath was foundto have undesirable fluctuations in fluorideiactivity.There fore, it is necessary thatthe bath contain at all times bothsodium and potassium and in order to efliect this in replenishing thebath it is necessary to add from 10 to 90% of the total mols of sodiumand potassium in the form of potassium and, preferably, the bath shouldbe replenished with 2 mols of potassium for each mol of sodium as thiswill prevent any tendency of the bath to produce powder on the coatingsunder heavy loading conditions. Analyses of precipitates formed by theoperating baths, when replenished in accordance with the teachings 'justgiven, indicate that the precipitate consists essentially of K NaAlF Inconnection with the'method of preparing and replenishing the coatingbathdescribed just above we have also discovered that a glossy, lacquer-likecoating can be produced if there be added to the bath, as formulatedwithin the teachings just given, from 1 to grams per liter of cryolite.

By way of example, but without limitation as to the specific ingredientsor concentrations, a bath falling under the purview of the presentinvention may be formulated in accordance with the following example:

Formula #1 Chromic acid grams 14.4 Aluminum dihydrogen phosphate (Al(HPO 50% solution mls 32.0 75% phosphoric acid mls 36.3 Sodium bifluoride(NaF.HF) "grams" 9.56 Potassium bifluoride (KFHF) do 6.02 Cryolite do3.17

Water, to make 1 liter.

Such a bath is capable of producing a green, lacquerlike coating whenused in a dip process at 100 F. with a treating time of approximately 5minutes. Insofar as its fluoride content is concerned, it should bereplenished in accordance with the instructions above given, namely,preferably with a material which, for each gram of fluoride introducedinto the bath there is included from 0.0065 to 0.0455 mol of sodium andpotassium. Preferably the molar proportion of a mixture of potassiumshould be from to 90% of the total of alkali metals added.

In replenishing such coating baths, replenishment with respect tophosphate, hexavalent chromium, and adjustment of acidity (or pH) iscarried out in accordance with the principles previously established asset forth in the patents referred to above.

Replenishment with fluoride, however, in accordance with the presentinvention must be accompanied by suitable replenishment with a mixtureof alkali metals including both sodium and potassium in the proportionsset forth above.

It has been found desirable, though not essential, for ease of bathcontrol, to add the alkali metal and fluoride in a replenishing materialseparate from the other ingredients. A suitable replenishing materialfor these ingredients is the following: 7

Formula #2 Grams Sodium bifluoride 44.2 Potassium bifluoride 55.8

.Other formulations suitable for fluoride and alkali metal replenishmentare the following:

4 Formula #4 Sodium fluoride 51.8 Potassium bifluoride 48.2

Formula #5 Sodium fluoride 49.7 Potassium bifluoride Q. 46.3 Cryolite 4Formula #6 Percent Sodium oifluoride 28.4 Potassium bifluoride 71.6

As previously stated, under very heavy bath loading conditions,replenishment of the alkali metal content of the bath should be made byreplenishing the bath with approximately 2 mols of K for every mol of Naadded. That this is highly desirable is illustrated by the fact that acommercial bath was made up in accordance with the example of Formula #1above given and was periodically replenished in respect to the fluoridecontent by using Formula #5. Under normal operating conditions thisworked quite well for several. months. However, when the bath wassubjected to heavy bath loading, its coating ability became somewhatunmanageable. When this occurred replenishment was changed from Formula#5 to Formula #6 (sodium bifiuoride 28.4% and 71.6% potassiumbifluoride). After this was done the bath again became manageable andproduced excellent coatings under extremely heavy loading conditionssuch as 10,000 sq. ft. per hour.

While the admixture of Formula #2 is primarily useful in effectingreplenishment of the fluoride content of the coating bath it should beunderstood that it will serve also as an excellent source of fluoride ininitially making up a new coating bath.

Examples of other useful coating baths falling into the purview of thepresent invention are as follows:

Formula 7 CrO grams 14.4 KEHF do 5.71 NaF.HF do 4.54 Na AlF do 4.24 Al(HPO (50% solution) mls 16.0 H 1 0 75% mls 36.3 Water, to make 1 liter.

Formula #8 Same as above except no Na AlF Formula #9 CrO grams 14.4KFl-IF do 2.85 NaF.HF do 2.27 Na AlF do 2.12 Al(H P0 (50% solution) mls8.0 H PO 75% mls 36.3 Water, to make 1 liter.

Formula #10 G0,, grams 14.4 NaF do 6.47 KF do 8.95 Al(H P0 (50%solution) mls 16 H3PO4 mls Water, to make 1 liter.

Formula #11 CrO g ms" 14.4 NaF do 6.07 KFHF dn 5.65 Al(H PO (50%solution) m1s 16 H PO (75%) 1s-- 6.3

Water, to make .1 liter.

Formula #12 Q: g-ams-- 14.4 KF.HF do 6.17 NaF.HF o 4.90 HF (48%) mls5,72 Na AlF g ams 3.93 A1(H PO (50% solution) mls 34 P0 mls 36.3 Water,to make 1 liter.

Formula #13 CrO g ams" 14.4 NaF.HF "do"-.. 4.79 KF.HF do- 12.05 Na AlFdo.. 1.91 A1(H P0 (50% solution) mls 32 H PO (75%) mls 36.3 Water, tomake 1 liter.

ticularly indicated.

While forming no part of the present invention the phosphate andhexavalent chromium content of such baths may advantageously be restoredby suitable additions of a solution in accordance with the formulaappearing below:

Formula #14 Grams H PO (75%) 437 CrO 110 Water, to make 1 liter.

This application is a continuation in part of our earlier applicationSerial No. 637,876, filed February 4, 1957, now abandoned.

We claim:

1. A bath for the coating of aluminum having a pH between 0.8 and 45whose coating producing ingredients consists essentially of:

6 Ions of- Grams/ liter Fluoride 0.15 to 12.5 Hexavalent chromium(expressed as CrO 3.0 to Phosphate (P0 2.0 to 285 said bath beingfurther characterized by containing a mixture of sodium and potassium ina total concentration of from 0.0026 to 0.0526 of a mol for every gramof fluorine present with potassium constituting from 10 to there of andalso by containing dissolved aluminum to the point of equilibrium in thepresence of the other essential ingredients whereby any precipitatewhich may be formed will consist essentially of K NaA1F 2. The method ofboth making up and replenishing the bath of claim 1 insofar as itsfluorine content is concerned which consists in adding to the baththerequisite amount of fluorine and from 0.0026 to 0.0526 of a mol of amixture of sodium and potassium for each gram of fluorine added to thebath.

3. The method of coating a succession of pieces of aluminum whichcomprises successively treating the pieces with the bath of claim 1 andperiodically replenishing the bath by adding thereto the requisiteamount of fluorine and from 0.0026 to 0.0526 of a mol of a mixture ofsodium and potassium for each gram of fluorine with potassiumconstituting from 10 to 90% thereof, and replenishing the hexavalentchromium and phosphate as may be required to maintain the contentthereof as specified.

4. An admixture for use in preparing and replenishing a bath for coatingaluminum, said admixture consisting essentially of water solublefluorides of sodium and potassium, in which admixture, for each gram offluorine there are from 0.0065 to 0.0455 of a mol of a mixture of sodiumand potassium with the potassium constituting from 10 to 90% thereof.

5. The admixture of claim 4 in which the molar ratio of sodium andpotassium is approximately 1 to 2.

6. The bath oi: claim 1, wherein the sodium and potassium content isfrom 0.0065 to 0.045 for each gram of fluorine with the ratio of sodiumto potassium being about 1 to 2.

References Cited in the file of this patent UNITED STATES PATENTS2,438,877 Spruance Mar. 30, 1948 2,472,864 Spruance June 14, 19492,489,152 Panepinto 'Nov. 22, 1949 UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent No 2 936 254 May 10 1960 Nelson J.Newhard Jr et a1n It is hereby certified that error appears inthe-printed specification of the above numbered patent requiringcorrection and that the said Letters Patent should read as correctedbelow.

Column 3, line 43 strike out "of a mixture" and insert the same before"sodium" in line 42, same column 3.

Signed and sealed this 18th day of October 1960" (SEAL) Attest: KARL H,AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents

1. A BATH FOR THE COATING OF ALUMINUIM HAVING A PH BETWEEN 0.8 AND 4.5WHOSE COATING PRODUCING INGREDIENTS CONSISTS ESSENTIALLY OF: